Basic sRGB Math
The sRGB color space is based on the monitor
characteristics expected in a dimly lit office. It has been standardized by the International
Electrotechnical Commission (IEC) as IEC 61966-2-1. The official specification should be purchased from the IEC
to assure that you have the most recent and correct information. This information is provided as an
informative reference only.
The expected colors of the red, green, and blue
monitor phosphors and the white setting of an sRGB monitor are specified in
chromaticity values below. The
non-linearity or gamma of the monitor is 2.2. The formulas used to convert between sRGB and XYZ
tristimulus values are also included below.
|
CIE chromaticities for ITU-R BT.709 reference primaries and
CIE standard illuminant |
||||
|
|
Red |
Green |
Blue |
D65
White Point |
|
x |
0.6400 |
0.3000 |
0.1500 |
0.3127 |
|
y |
0.3300 |
0.6000 |
0.0600 |
0.3290 |
|
z |
0.0300 |
0.1000 |
0.7900 |
0.3583 |
Converting XYZ to sRGB
The red, green and blue phosphor chromaticities and D65 white
point results in the following relationship between D65 tristimulus values and
linear RGB values:
Any values greater than 1.0 or less than 0.0 are
clipped to 1.0 and 0.0 respectively.
The linear RGB values are transformed to nonlinear
sR'G'B' values as follows:
If R,G, B £ 0.0031308
R’sRGB
= 12.92 ´
R
G’sRGB
= 12.92 ´
G
B’sRGB
= 12.92 ´
B
else if R,G, B > 0.0031308
R’sRGB
= 1.055 ´
R(1.0/2.4) – 0.055
G’sRGB
= 1.055 ´
G(1.0/2.4) – 0.055
B’sRGB = 1.055 ´ B(1.0/2.4) – 0.055
The nonlinear sR’G’B’ values are then converted
into 8 bit integers by:
R8bit =
round(255.0 ´
R’sRGB)
G8bit =
round(255.0 ´
G’sRGB)
B8bit = round(255.0 ´ B’sRGB)
Converting sRGB to XYZ
The 8 bit integer sRGB values are converted to
floating point non-linear sR'G'B' values as follows:
R’sRGB = R8bit / 255.0
G’sRGB = G8bit
/
255.0
B’sRGB = B8bit
/
255.0
The nonlinear sR'G'B' values are transformed to
linear R,G, B values by:
If R’sRGB,G’sRGB, B’sRGB £ 0.04045
R
= R’sRGB ¤ 12.92
G = G’sRGB ¤ 12.92
B = B’sRGB ¤ 12.92
else if R’sRGB,G’sRGB, B’sRGB > 0.04045
R = ((R’sRGB
+ 0.055) / 1.055)2.4
G = ((G’sRGB
+ 0.055) / 1.055)2.4
B = ((B’sRGB
+ 0.055) / 1.055)2.4
Then convert to XYZ by:
R G B *
|
sRGB Viewing Environment Summary * |
|
|
Condition |
sRGB |
|
Display Luminance level |
80 cd/m2 |
|
Display White Point |
x = 0.3127, y = 0.3290 (D65) |
|
Display model offset (R, G and B) |
0.0 |
|
Display input/output characteristic |
2.2 |
|
Reference ambient illuminance level |
64 lux |
|
Reference Ambient White Point |
x = 0.3457, y = 0.3585 (D50) |
|
Reference Veiling Glare |
0.2 cd m-2 |
*Please refer to the official IEC 61966-2-1 specification for more of the viewing environment details and descriptions.